Some strains of Bacillus proteus obtained from human lesions are pathogenic for rabbits, rats, and guinea pigs. There is good evidence that these strains are also pathogenic for man.

All cultures obtained from sources other than human infections were non-pathogenic for the laboratory animals.

A non-pathogenic culture may be made pathogenic by the use of aggressins or by inoculation into the anterior chamber of the eye.

Proteus cultures lose their virulence rapidly when grown on artificial media.

The lesions produced in animals are either simple abscesses, proliferative lesions, or a mixed exudative and proliferative lesion.

The proliferative lesions consist mainly of epithelioid cells apparently of connective tissue origin. No giant-cells of the Langhans type are present.

The histological type of the lesion does not depend upon the strain employed. Neither does it bear any relation to the clinical severity of the case.

The ability to produce the characteristic lesions has no necessary connection with the toxicity of the bacteria.

The proteus bacteria probably play a more important part in human pathology than is generally believed.

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